1. Field of the Invention
This invention relates to a circuit configuration to operate a gas discharge lamp, in particular a low pressure-discharge lamp, and to a method of operating a gas discharge lamp.
2. Description of the Related Art
In the design of electronic chokes to operate gas discharge lamps essentially two requirements have to be met. On the one hand the electronic choke is intended to facilitate reliable and effective lamp operation, on the other hand the choke and the lamp should not interfere with the function of other apparatus or have any effects on the surroundings.
In order to achieve as high as possible lamp efficiency, gas discharge lamps are normally operated at high frequencies in a range of 20 to 50 kHz. At these frequencies the electron density in the plasma of the fluorescent lamp is no longer able to follow the time variation of the current, so that an essentially time-constant, average electron density ensues. In this case in the event of a crossover of the mains supply no further current gap arises to reignite the lamp, so that compared to lamp operation at a normal mains frequency, e.g. 50 Hz, with a constant stream of light a reduction in electrical consumption of 8 to 10% can be achieved.
On the other hand as a result of the high frequency operation of the lamp high-frequency alternating fields arise, in which an adverse effect on the surroundings cannot be totally ruled out. For example these alternating fields may lead to complaints from people in the vicinity, for example of headaches or similar conditions. In particular, regarding gas discharge lamps which are shaped in the form of long tubes, steps must be taken to limit their radiated field strength.
The easiest way to prevent high-frequency fields arising is to again reduce the operating frequency of the lamp and to operate ideally with DC current. However in practice it is not possible to operate the lamps using pure DC voltage, since gas discharge lamps have a negative characteristic. This canxe2x80x94from an economic point of viewxe2x80x94only be reasonably counteracted by using a choke. Operating with pure DC voltage in addition has the disadvantage that the electrodes of the lamp wear out unevenly, which can be seen in irregular discoloration or blackening of the glass around the electrodes.
A circuit configuration, which on the one hand allows high-frequent lamp operation and on the other hand produces high-frequent alternating fields to a relatively limited extent, is described in WO 86/04752. In this case the lamp is arranged in a full bridge circuit comprising four controllable switches and actuated in such a way that in a first operating phase two switches lying diagonally opposite each other are opened, while one of the two switches of the second bridge diagonal is permanently closed and the other is pulsed at high frequency. During this time current flows through the lamp essentially in one direction. In order to prevent harmful deposits occurring over a period of time on the electrodes, the four switches are actuated in a second operating phase after a certain period so that the two previously permanently opened switches are closed or pulsed at high frequency, while the other two switches are opened, which effectively corresponds to pole reversal of the lamp. In this way the lamp is operated with a low-frequency voltage signal, which is superimposed with high-frequency oscillation. Since however the amplitude of the high-frequency oscillation is relatively low, high-frequency alternating fields are produced and diffused into the atmosphere only to a relatively minimal extent.
It is the aim of the present invention to provide a full bridge circuit configuration to operate a gas discharge lampxe2x80x94preferably a low pressure-discharge lamp, through which the lamp is operated with a high degree of efficiency and which, on the other hand, facilitates efficient lamp starting. At the same time the diffusion of high frequency fields should be reduced.
This aim is achieved by a circuit configuration which has first, second, third and fourth controllable switches. The first and second switches are arranged in series with each other between DC voltage supply terminals, and the second and fourth switches are also arranged in series with each other between the DC voltage supply terminals. A first nodal point between the first and second switches and a second nodal point between the third and fourth switches are connectable, respectively, to opposite terminals of a gas discharge lamp. A control circuit is connected to open and close the switches according to a first state, a second state and a start state. The first state and second states occur alternately to each other at first frequency. Also, in the first state, the second and third switches remain open and the first switch remains closed while the fourth switch opens and closes at a second frequency which is higher than the first frequency. In the second state, the first and fourth switches remain open and the third switch remains closed while the second switch opens and closes at the second frequency. In the start state, the fourth switch remains closed, the third switch remains open and the first and second switches open and close alternately at a third frequency.
With this circuit, the lamp is operated with a full bridge circuit in such a way that switching over takes place between two bridge diagonals in normal operation with a first frequency, whereby each time the switches of the one bridge diagonal are opened and the switches of the other bridge diagonal are closed or are pulsed with a second frequency, which is greater than the first frequency. Here also the gas discharge lamp is therefore operated in normal operation with a voltage signal which is composed of a first low-frequent signal with a relatively high amplitude as well as a second high-frequent signal with a low amplitude. According to the invention, however, to start the lamp the switch of a half-bridge connected to the DC voltage source of the full bridge circuit is opened, while the second switch of this half-bridge is closed and at the same time the two switches of the other half-bridge are high frequent opened and closed. Therefore the lamp is supplied for starting with an AC voltage the amplitude of which is greater than the amplitude of the low-frequent voltage signals in normal operation and the frequency of which is less or equal to the second high frequency. Therefore the lamp when starting is operated for a short time with a strong high frequent signal, while it is supplied after ignition with a DC voltage changed over to low frequency, which is only superimposed by the high frequency. In this way considerably better and smoother lamp starting is achieved. Also at the same time however very low values of radiation possibly arising are assured which does not lead to any ill-health effects on people in the vicinity, since the high frequency-signal overlays the lamp DC current to a correspondingly minimum extent. In addition through the periodic switch-over of the lamp current to low-frequency, for example in the mHz to Hz range, it is guaranteed that the electrodes wear out equally and no irregular discoloration or blackening of the lamps will occur at one end.
Further developments of the invention are described and claimed herein. Since a circuit configuration according to the invention ensures reliable lamp operation and at the same time diffusion of high frequent alternating fields is considerably reduced, very high frequencies can be used for the switches pulsed in normal operation. In particular it is conceivable to pulse the switches at a frequency of above 1 MHzxe2x80x94preferably even in the range between 2.2 MHz and 3.0 MHz. This is roughly 20 to 100 times higher than the present normal operating frequencies of between 20 kHz and 50 kHz. In this way the possibility of increasing lamp efficiency is created. The frequency range between 2.2 MHz and 3.0 MHz in this case is especially favorable, since standards of most jurisdictions allow in this frequency range, a somewhat increased diffusion of interference on the mains supply in the form of upper waves.
Also, surprisingly it has been shown that low pressure-discharge lamps possess extremely good striking characteristics at these maximum frequencies. In this case the pulse sequence according to the invention is also particularly suitable for starting lamps in the megahertz-range. Due to the good striking characteristics even pre-warming of the lamp-electrodes can possibly be dispensed with and the lamp instead can be started immediately from cold. This again means that a relatively simple circuit configuration can be used.
If in fact lamp operation in the megahertz-range is selected, another advantage is that the capacitance and inductance values of some of the component parts used in the circuit configuration can be reduced in such a way that instead of using discrete structural elements these can be fully integrated. In particular passive structural elements such as chokes and capacitors, which for example can be component parts of a serial resonance circuit arranged in the bridge arm, or transformers are a good case for integration so that a considerable reduction in the dimensions of the overall circuit can be achieved. The structural elements can for example be integrated as part of a multi-layer printed circuit board. Preferably the multi-layer circuit is produced in the form of a LTCC (Low Temperature Cofired Ceramic) structure which consists of several low sintering ceramic layers or films placed on top of each other, between which circuit tracks, capacitor-forming dielectric layers or similar are located.
According to a further aspect of the invention, there is provided a novel method of operating a gas discharge lamp. This novel method involves supplying the gas discharge lamp in normal operation with an AC voltage, which comprises a first signal having a first frequency and a first amplitude, onto which a second signal having a second frequency and a second amplitude is superimposed. The second frequency is greater than the first frequency and the first amplitude is greater than the second amplitude. The method also involves supplying the gas discharge lamp in a start phase with an AC voltage, the amplitude of which is greater than the first amplitude and the frequency of which is equal to or less than the second frequency.